Solid matter micro-encapsulated by an aminoplast and method for producing the same

ABSTRACT

Micro-encapsulated particles of solid material are described and a quasi-continuous method for the production thereof, in particular those with a water-insoluble character or in a hydrophobised form taking into account in particular an ecologically and economically favourable recycling of the used media and the resultant smallest possible amount of waste products, such as waste water, polymer residues inter alia while simultaneously ensuring the coating even of exposed parts of the solid bodies, such as crystal tips, grooves and the like which, due to the hydrodynamic conditions at these points, can only be coated inadequately with standard methods.

[0001] The invention relates to particles of solid materialmicro-encapsulated by an aminoplast and to a quasi-continuous method forthe production thereof taking into account in particular environmentallyrelevant aspects by means of controlled recycling of the media used.

[0002] Micro-encapsulated particles of solid material are used inagriculture and forestry, in hygiene products, in the human andveterinary field, in the cosmetic industry, in products of the foodindustry, packaging industry, construction industry and also enamel andpaint industry, in the environmental protection sphere and in many othercommercial products. Their use occurs as dispersions, free-flowingpowders or direct incorporation in other materials, in particular in thepolymer sector by injection-moulding, sintering, extrusion or othercurrent shaping methods.

[0003] The micro-encapsulation by means of amino resins is described inthe literature in many ways. Thus for example EP 0 017 386 teaches theuse of substances micro-encapsulated by amino resins in the field of theproduction of carbon paper. A specific modification of the wall-formingamino resin used constitutes as a rule a main emphasis of theinventions, which modification has been undertaken primarily withrespect to the core material, as U.S. Pat. No. 4,525,520 teaches. Thecombination with additive materials which are inert relative to theamino resin is also known from EP 0 532 462.

[0004] Generally, primarily liquid components are micro-encapsulated andthis relates very particularly to the use of amino resins aswall-forming materials. Usually coating methods are used for solidmaterials, which methods are based primarily on physical principles,such as for example the Wurster technique and other spray-coatingmethods, coating or granulating methods (cf. B. H. Kaye“Microencapsulation” KONA 10 (1992) 65) or complex methods are appliedusing supercritical solvents (Ind. Eng. Chem. Res. 26 (1987) 2298).

[0005] Direct micro-encapsulation of solid materials is described in EP0 940 171 which use complex polymer/monomer compositions.

[0006] The problem with micro-encapsulation of solid materials isproduced by their hydrodynamic behaviour, in particular if this concernsirregular shapes or even needle-shaped, crystalline products. At theexposed points of these materials (needle tips, sharp edges on crystals,cavities or pores or the like), there are formed high flow rates andflow turbulences which prevent a qualitatively tolerable coating of thematerial in this region. Because of the high flow rates, theaccumulating prepolymer colloids are rapidly removed again. A reductionin the flow rate is however not generally possible since agglomeration,aggregation and coalescence occur to a greater extent. Continuousmicro-encapsulation methods are also known from U.S. Pat. No. 4,454,083and from U.S. Pat. No. 4,105,823 in which amino resins are also used asco-components. In U.S. Pat. No. 4,898,696, a method is described whichoperates with water-soluble high polymers with strong acid groups whichserve as hardening components for melamine-formaldehyde pre-condensates.It is hereby a disadvantage that the high polymer product used is eitherincorporated into the capsule wall or tends to precipitate or flocculateduring the generation process and is then difficult to separate from thecapsule product. One often observes then also agglutination of theindividual particles, agglomeration and aggregate formation. The objecttherefore underlying the invention is to develop micro-encapsulatedparticles of solid material and a quasi-continuous method for theproduction thereof, by means of which these described defects areavoided.

[0007] This object is achieved by the generic particles of solidmaterial with the features of claim 1 and the generic method for theirproduction with the features of claim 4. Uses according to the inventionof the method are characterised by the features of claims 21 and 22. Therespective sub-claims contain advantageous developments.

[0008] According to the invention, the particle of solid materialmicro-encapsulated by an aminoplast comprises a particle, the surface ofwhich is encapsulated by an aminoplast. The aminoplast thereby comprisesthe condensation product of partially etherified amino resinpre-condensates. At least one of these amino resin pre-condensates hasthereby been modified by co-condensation with an ethoxylated fattyamine, as a result of which an increased surface activity of thesecomponents is achieved. Due to the high surface activity, theencapsulation of the particle of solid material is therefore alsopossible at exposed positions with high homogeneity and uniformity.

[0009] The thickness of the encapsulation, i.e. the layer thickness ofthe aminoplast, is preferably more than 50 nm and less than 300 nm.

[0010] A preferred embodiment of the invention proposes that the corehas been hydrophobised. Furthermore, it is also possible that ahydrophobisation agent is contained in the aminoplast itself. However,the embodiment is preferred in which a hydrophobisation has beenimplemented before the encapsulation.

[0011] The advantage of this embodiment can be seen in the fact that aseries of solid materials are sensitive to hydrolysis. Remedial actioncan now be taken here due to the hydrophobisation step. It is herebypreferred if the solid materials are treated preferably withpolydimethylsiloxanes and particularly preferred with those which havehydroxy end groups in the form of the commercially available product ordissolved in a diluted form in a suitable organic solvent which iscompatible also with the solid material. A further suitablehydrophobisation agent is represented by waxes and particularlypreferred by paraffin waxes with various melting points between roomtemperature and 130° (dependent upon the solid material to behydrophobised), the melt of the wax being applied preferably for thehydrophobisation. However, the use of dissolved waxes instead of themelt is also possible here in many cases. Especially for materials witha high proportion of meso- and macro-pores, liquid paraffins andparaffin oils are also preferably suited. The proportion ofhydrophobisation agent applied to the solid material can be 0.05 to 50%by weight relative to the solid material, preferably 0.1 to 20% byweight and particularly preferred 0.5 to 5% by weight.

[0012] The method for producing the micro-encapsulated particles ofsolid material is divided into the following method steps:

[0013] a) Micro-encapsulation process,

[0014] b) Maturing process (post-curing),

[0015] c) Separation method including drying of the micro-encapsulatedmaterial,

[0016] d) Reprocessing and regeneration of the continuous medium(original solution).

[0017] Water and the reactants are introduced thereto in a suitablereaction vessel, equipped with suitable agitation and measuringtechnology and heating, regenerated original solution from the processbeing able to be introduced, in part or also exclusively, instead ofwater.

[0018] For the condensation into the aminoplast, generally two types ofamino resin pre-condensates A and B are used. The pre-condensate Atherefore concerns a melamine resin pre-condensate which has beenpartially methylated, with a comparatively high methylol group contentand a low imino functionality. A further characteristic feature of thepre-condensate is its arbitrary water compatibility or dilutability. Thepre-condensate B ensures a surface-active character of the system whichis substantially increased with an increasing degree of condensation andconsequently ensures the coating of the hydrodynamically disadvantagedsolid material portions. Derived from the synthesis specification B, itis also possible to use a pre-condensate from DE 198 14 880. It ischaracteristic and indispensable for the configuration of thequasi-continuous operation that the adjusting equilibrium between thepolymer deposited on the solid material and the pre-polymer remaining inthe solution in the reaction system is capable of no noteworthy furtherpolycondensation. Only by separation of the micro-encapsulated solidmaterial does the reaction proceed slowly. This is effectively retardedby renewed addition of pre-condensate (building up). The regeneratedmedium is supplied to a renewed micro-encapsulation synthesis.

[0019] In the proposed system, it is necessary to use bufferingcatalysts which ensure a constant pH value of the system. Conditioned bythe melamine used in the pre-condensate, a protonation of the melaminecan be effected even at a pH of approximately 4.0, the buffering acidcatalyst then imparting the equilibrium here.

[0020] Dependent upon the reaction temperature and the pH value of themedium (cf. Table 1), the dwell time in the reaction vessel is between 5and 30 minutes.

[0021] Subsequently, the micro-encapsulation dispersion is transferredinto the maturing reactor by the use of suitable pumps or better bymeans of gravity. The maturing reactor is filled during moderateagitation, the inlet being attached conveniently on the vessel base andthe outlet to the separation plant being attached in the upper vesselregion. The agitator in the maturing reactor should have a discharge ofthe material in the direction of the vessel base. Heating of thematuring reactor is in fact favourable but is not absolutely required inevery case. The size ratio of reaction vessel to maturing reactor shouldnot fall below the factor 10 in order that a sufficiently high dwelltime of the generated micro-encapsulated product in the maturing reactoris ensured. Instead of the size of the maturing reactor, it is of coursealso possible to use a vessel cascade, a loop reactor or also a flowpipe which does not impair the required contents. The average dwell timeof the micro-encapsulated products in the maturing reactor should notfall below 30 minutes.

[0022] After conclusion of the maturing process, separation of theproduct from the liquid (original solution) is effected for examplecontinuously via a trailing blade centrifuge or quasi-continuously via asuction filter cascade with discharge of the product.

[0023] Next drying takes place in the normal manner, the dryingtemperatures being able to move within the range of room temperature to150° C. dependent upon the core material. In the case of application ofdispersions, separation and drying processes can be partially dispensedwith and the dispersion can be used directly after possibly furtherformulation.

[0024] When using the separation methods, the pH value and theconductivity in the separated original solution after build-up of thevolume is determined in a conventional suitable manner and also the acidcontent is determined titrimetrically. When using surface-activeadditives or protective colloids, these can also be determined bysuitable methods.

[0025] In laboratory tests, the equilibrium concentration of amino resincombinations was determined (e.g. nitrogen determination, FTIRspectroscopy and the like). This can be dependent upon the core materialand also upon the type and concentration of catalyst used and should bedetermined by changing these parameters before application in laboratorytests.

[0026] The regenerated original solution is then transferred into thesupply vessel provided for this purpose and is therefore available forre-use in the micro-encapsulation process.

[0027] If the necessity for re-washing of the product is present, thisshould be effected in a semi-continuous operation on the trailing bladecentrifuge or suction filter, a first proportion of approximately 20% ofthe total quantity being able to be supplied likewise to the originalsolution. A minimal strain on sewage or on the environment is henceensured. When connecting two reaction vessels with optional actuation ofthe individual vessels, the implementation of the quasi-continuousprocess is possible.

[0028] A melamine-formaldehyde pre-condensate is used preferably for thepolycondensation, 5 mol formaldehyde per mol melamine being used.

[0029] Between 2 and 4 methylol groups of the pre-condensate A arepreferably etherified with methanol, 3 methylol groups are particularlypreferred.

[0030] The pre-condensate A can preferably be stabilised by the additionof substances containing amide groups and also by alcanol amines.

[0031] The co-condensation of the pre-condensate B is implementedpreferably by adding 5 to 50%, particularly preferred 15%, of anethoxylated fatty amine. The fatty amine used has preferably a KOHnumber between 80 and 140 and particularly preferred between 110 and120. The temperature for this co-condensation lies thereby preferablybetween 20 and 80° C. and particularly preferred 60° C., the reactionduration being no shorter than 30 minutes and no longer than 120minutes, the reaction being implemented preferably for 60 minutes.

[0032] It is advantageous if the mass ratio relative to 100% solid resincontent between the pre-condensates A and B is between 1:0.2 and 1:10,particularly advantageous however is a mass ratio relative to 100% solidresin content of 1:1.

[0033] Organic acids, such as formic acid, acetic acid, ascorbic acid,citric acid and other organic acids with sufficiently high watersolubility can be used optionally. Equally, use of acid combinations isalso indicated, such as citric acid/phosphoric acid, ascorbicacid/hydrochloric acid, ascorbic acid/formic acid and the like or theuse of combinations of organic acids with acidic salts, such as aceticacid/acetates, ascorbic acid/tartrates inter alia.

[0034] The coated particles of solid material produced according to themethod according to the invention relate in particular to metallicparticles, such as metal flakes, metal oxides, such as aluminium oxide,titanium dioxide, zinc oxide, iron oxides or other metals and alsocrystalline water-insoluble or water-soluble previously hydrophobisedmaterials, colourants, marker colours or also magnetisable particleswhich are used in the most varied of industrial and commercial fields.

[0035] The subject according to the invention is now intended to beexplained by way of example with reference to the following examples.

EXAMPLE A

[0036] Production of a Modified, Partially Etherified Melamine Resin

[0037] Into a suitable agitated vessel, equipped with agitator, heatingand cooling device and vacuum distillation device, there is added forexample 4450 ml 30% by weight aqueous formaldehyde, heated to 80° C. andthe pH value is set at pH 9 by means of a concentrated caustic sodasolution. Subsequently, 1000 g melamine are introduced. The temperaturerises to approximately 90° C. through the exothermic reaction course.After the solution has become clear (dissolution of the melamine withmethylol formation), it is rapidly cooled to 60° C. The formingcrystalline precipitation (methylol melamine) is diluted with 4300 mlmethanol and subsequently the pH value is adjusted downwards withhydrochloric acid to values of 1 to 2. The reaction is agitated furtheruntil the solution becomes clear. Subsequently, the pH value is set at 8to 9 with triethanol amine. At a temperature of 60° C., 2500 ml of amethanol-water mixture are then distilled off at reduced pressure. Theresultant resin solution has a solid material content of approximately41% and is stabilised by the addition of 166 g urea. The synthetic resinis stable in room conditions for at least 6 months.

EXAMPLE B

[0038] Synthesis of a Modified, Partially EtherifiedMelamine-Formaldehyde Resin with Surface-Active Character

[0039] In a heatable agitated vessel equipped with agitator and refluxcooler, for example 500 g of a commercially available, highlyhydroxymethylated amino resin pre-condensate etherified partially withmethanol on a melamine basis, for example Cymel 373 of Cytec DeutschlandGmbH are introduced.

[0040] Subsequently there is introduced 100 g of an ethoxylated fattyamine, e.g. ethoxylated tallow fat amine with an alkali number ofapproximately 115 mg KOH/g and the pH value is set at 7.5 to 8.5 bymeans of 2N aqueous citric acid and is heated with reflux over 60minutes at 60° C. After cooling a syrupy honey yellow product isobtained which can be stored for over at least 3 months.

EXAMPLE C

[0041] Synthesis of Micro-Encapsulated Solid Material

[0042] In a reactor equipped with corresponding agitator technology, forexample 60 l water are introduced and heated to 60° C. From suitablesupply vessels, there are incorporated successively 7 litres 2N aqueouscitric acid solution, 6 litres of the melamine resin solution fromexample A and 3 litres of the melamine resin solution from example B.The point in time of the resin addition is registered. The two resinsolutions can also be mixed in advance. After a pre-condensation time of5 minutes, for example 10 kg of a metal oxide powder (e.g. titaniumdioxide with a particle size distribution d₅₀=5 μm, d₅₀=10 μm, packingdensity 1,000 g/dm³) are introduced rapidly during agitation. In thepresent case agitation takes place with a turbine agitator at 1000 rpm.This is left to agitate further for 10 minutes at a temperature of 60°C. and then is transferred particularly favourably according to thegravity principle into the maturing vessel. Here the product is furtheragitated at a temperature of 60° C. The product discharge from thematuring vessel is measured such that an average dwell time of theindividual charge of 120 minutes results. The removed product isde-watered continuously via a trailing blade centrifuge orquasi-continuously via a suction battery. The original solution isreturned into the supply vessel provided for this purpose and is re-usedduring synthesis after regeneration. The discharged moist product isdried in a suitable manner and possibly graded. Using the citedoperation there are obtained titanium dioxide particles with an aminoresin coating with a thickness of on average 180 nm.

1. Particles of solid material micro-encapsulated by an aminoplast,characterised in that the aminoplast is the condensation product frompartially etherified amino resin pre-condensates, of which at least onecomponent has a high surface activity due to a co-condensation with anethoxylated fatty amine with a KOH number between 80 and
 140. 2.Particles of solid material according to claim 1, characterised in thatthe thickness of the micro-encapsulation on the entire particle is morethan 50 nm.
 3. Particles of solid material according to one of thepreceding claims, characterised in that the thickness of themicro-encapsulation on the entire particle is less than 300 nm. 4.Particles of solid material according to one of the preceding claims,characterised in that the surface of the particle is hydrophobisedand/or a hydrophobisation agent is contained in the aminoplast. 5.Method for production of particles of solid material micro-encapsulatedby aminoplasts, characterised in that a) the particles of solid materialare introduced into a reaction vessel in which an acid-catalysedpolycondensation occurring in aqueous phase is implemented by at leasttwo amino resin pre-condensates, of which at least one has been modifieddue to co-condensation with an ethoxylated fatty amine with a KOH numberbetween 80 and
 140. b) a post-curing of the polymer is effected in alarger collection vessel with continuous removal option of the mediumand c) the micro-encapsulated particles of solid material are separatedfrom the original solution and are dried and d) the original solution isre-processed and re-supplied to the method.
 6. Method according to claim5, characterised in that a component A from the amino resinpre-condensates is partially etherified with methanol and a component Bis modified in a surface-active manner by co-condensation with anethoxylated fatty amine.
 7. Method according to at least one of theclaims 5 or 6, characterised in that the component A has 5 molformaldehyde per mol melamine.
 8. Method according to at least one ofthe claims 5 to 7, characterised in that at least 2 and at most 4methylol groups are etherified with methanol in the case of component A.9. Method according to at least one of the claims 5 to 8, characterisedin that three methylol groups are etherified with methanol in the caseof component A.
 10. Method according to at least one of the claims 5 to9, characterised in that the component A has been stabilised with asubstance containing amide groups and with an alcanol amine.
 11. Methodaccording to at least one of the claims 5 to 10, characterised in thatthe component B is produced by co-condensation with 5 to 50% of anethoxylated fatty amine.
 12. Method according to at least one of theclaims 5 to 11, characterised in that the component B is produced byco-condensation with 15% of an ethoxylated fatty amine.
 13. Methodaccording to at least one of the claims 5 to 12, characterised in thatthe ethoxylated fatty amine has a KOH number between 110 and
 120. 14.Method according to at least one of the claims 5 to 13, characterised inthat the co-condensation of component B is implemented at temperaturesbetween 20 and 80° C.
 15. Method according to at least one of the claims5 to 14, characterised in that the co-condensation of component B isimplemented at a temperature of 60° C.
 16. Method according to at leastone of the claims 5 to 15, characterised in that the duration of theco-condensation of component B is between 30 and 120 min.
 17. Methodaccording to at least one of the claims 5 to 16, characterised in thatthe duration of the co-condensation of component B is 60 min.
 18. Methodaccording to at least one of the claims 5 to 17, characterised in thatthe mass ratio relative to the solid resin content of components A to Bis between 1:0.2 and 1:10.
 19. Method according to at least one of theclaims 5 to 18, characterised in that the mass ratio relative to thesolid resin content of components A to B is 1:1.
 20. Method according toat least one of the claims 5 to 19, characterised in that there are usedas catalysing acid, organic acids, such as e.g. citric acid, ascorbicacid, acetic acid or mixtures thereof.
 21. Method according to at leastone of the claims 5 to 20, characterised in that there are used ascatalysing acid, mixtures of organic acids and inorganic acids. 22.Method according to at least one of the claims 5 to 21, characterised inthat the catalyst is active in a pH range of 2 to
 7. 23. Methodaccording to at least one of the claims 5 to 22, characterised in thatthe catalyst is active in a pH range of 3.5 to
 4. 24. Method accordingto at least one of the claims 5 to 23, characterised in that 0.05 to 10%by weight hydrophobisation agent relative to the weight of the corematerial are applied to the particle of solid material and in that theencapsulation is subsequently implemented.
 25. Use of the methodaccording to claims 5 to 24 for producing micro-encapsulated metal ormetal oxide particles.
 26. Use of the method according to claims 5 to 24for micro-encapsulation of crystalline substances.